How is the HAVING clause used with the GROUP BY clause in SQL?

The HAVING clause in SQL is used with the GROUP BY clause to filter the results of the grouping.

In SQL, the GROUP BY clause is used to group rows that have the same values in specified columns into aggregated data. It is often used with aggregate functions (COUNT, MAX, MIN, SUM, AVG) to perform calculations on each group of rows. However, there are times when you want to filter the results of your grouping, and this is where the HAVING clause comes into play.

The HAVING clause was added to SQL because the WHERE keyword could not be used with aggregate functions. The WHERE clause is used to filter rows before they are grouped and aggregated, while the HAVING clause is used to filter the results after they have been grouped and aggregated. This makes the HAVING clause very useful when you want to filter the results of your GROUP BY clause based on a condition.

For example, let's say you have a table of sales data and you want to find out the total sales for each salesperson, but you're only interested in those salespeople who have made more than £1000 in total sales. You could use the GROUP BY clause to group the sales data by salesperson and the SUM function to calculate the total sales for each salesperson. Then you could use the HAVING clause to filter out the salespeople who have made less than £1000 in total sales.

Here's what the SQL statement might look like:

```
SELECT Salesperson, SUM(Sales) as TotalSales
FROM SalesData
GROUP BY Salesperson
HAVING SUM(Sales) > 1000;
```

In this SQL statement, the GROUP BY clause groups the sales data by salesperson, the SUM function calculates the total sales for each salesperson, and the HAVING clause filters out the salespeople who have made less than £1000 in total sales.

In conclusion, the HAVING clause in SQL is a powerful tool that allows you to filter the results of your GROUP BY clause based on a condition. It's particularly useful when you're working with aggregated data and you want to filter the results based on a condition that involves an aggregate function.